Dimethylformamide-sulphur trioxide (DMF-SO 3 ) complex was found to be more suitable for tyrosine sulphation than pyridine-sulphur trioxide (Pyr-SO 3 ) complex, the most commonly used sulphur trioxide complex for sulphation. The work-up after sulphation using DMF-SO 3 was also easier than with Pyr-SO 3 . The usefulness of DMF-SO 3 complex was demonstrated through the synthesis of two tyrosine sulphate [Tyr(SO 3 H)]-containing peptides, leucine-enkephalin sulphate and leucosulfakinin-II.